The present invention relates to a light detecting circuit which can cancel or suppress a noise component superposed on a bias voltage which might otherwise adversely affect the detected light signal.
FIGS. 1A and 1B shows conventional light detecting circuits. In these figures, reference numeral 1 designates a light detecting photodiode; 2, a differential amplifier; and 3, a noise filter. In order to operate the light detecting photodiode 1, it is necessary to apply a bias voltage V.sub.b thereto; however, when the light detecting photodiode 1 is operated, a noise component having a waveform as shown at the top of FIG. 1A is often superposed on the bias voltage V.sub.b. The noise is applied to an input terminal of the differential amplifier 2 through a parasitic capacitor C.sub.t, typically about 3 pF, of the photodiode 1 and is amplified together with the received optical signal by the differential amplifier 2.
Such noise adversely affects the detection of the optical signal received by the photodiode 1. For instance, if the noise voltage is 10 mV and the parasitic capacitor C.sub.t of the photodiode is 3 pF, the noise current flowing through the parasitic capacitor C.sub.t is about 1.8 .mu.A. If the optical input has a power level of 1 .mu.W, the corresponding signal current is 0.6 .mu.A. Therefore, it can be understood that the noise current is much larger than the signal current. Accordingly, in the conventional light detecting circuit, it is necessary to apply the bias voltage V.sub.b through the noise filter 3 which is used for a bias circuit, as shown in FIG. 1B. However, the circuit indicated in FIG. 1B is disadvantageous in that it is space consuming, particularly, in that the inductor required thereby cannot be readily fabricated in integrated circuit form.
Accordingly, it is an object of the present invention to provide a light detecting circuit in which a noise component superposed on a bias voltage is eliminated without the use of a filter circuit.